When choosing a heater, it's important to know what volts, watts, and BTUs mean. Smaller space heaters will typically need to be fed by a 120 volt supply, while industrial and garage heaters will require a 240 volt supply. Learn more about volts, watts, and BTUs below.
Voltage 

Voltage is measured as the root mean squared value between two slots or holes in an electric outlet of the sort typically mounted in a wall. The purpose of an electrical outlet is to provide an area for you to plug in various electrical appliances in order to supply them with power. Most standard power outlets in the United States are of the 120 volt variety, which have spots for two plugs, and are rated for 15 amps. Therefore, for these standard type outlets, you will be able to use any of the standard 120 volt heaters. However, it is very important to keep in mind that these ratings cannot be exceeded, as the outlet could be damaged or a fire could be started. In addition, if your outlet is only rated for 15 amps, you cannot have a 14 amp heater and other appliances running at the same time on the same circuit, or it will trip the breaker. 
With that in mind, larger appliances are powered through 240 volt plugs, and cannot be powered through standard plugs. If they were fed by a 120 volt supply, they would need more than 20 amps of current running through the wires. In other words, the voltage from one hot wire to the other (called line to line voltage) is 240 volts, and the voltage from either wire to the neutral centertap (called line to neutral voltage) is half that, or of the 120 volt variety as discussed above. These 240 volt outlets can also be found in garages, building sites, and other industrial areas, and this is done to provide the safer, 120 volt power to outlets that need to supply less than 2.4 kilowatts (as opposed to appliances like clothes dryers, which can pull 4 kilowatts).


120V Plug 
240V Plug 
While a 240 volt garage heater is significantly more efficient and powerful than a 120 volt space heater, this may not be necessary for your home. Though most space heaters are 120 volts, some wall heaters designed for home use are rated at 240 volts and are just as powerful as garage heaters.
Watts 
A watt is an SI (metric) unit of power equal to one joule of energy per second. In terms of wattage, almost all 120 volt space heaters are rated up to 1500 watts at the maximum setting, and there is a 10/1 ratio of watts to square feet heated. Therefore, a 1500 watt space heater (120 volts) will be able to heat an area of 150 square feet, and a garage heater rated at 5,000 watts should be able to comfortably heat an area over 400 square feet. 
BTUs 
In terms of BTUs, or British Thermal Units, this is also a unit of energy that is used globally in heating and air conditioning industries, but is not commonly used scientifically. Nonetheless, in North America, the term BTU describes the energy value of fuels, and is also used to describe the power of heating and cooling systems such as heaters. When used as unit of power, BTUs per hour is the commonly used term, and refers to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. To convert watts to BTUs and vice versa, here are the conversion ratios:
1 watt = approximately 3.41 BTU/hour
1000 BTUs/hour is approximately 293 watts 
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